In the following description of the invention the term "thermal treatment" is intended to include such processes as thermal cracking, visbreaking and coking, terms of art to those skilled in the art of petroleum refining.
The term "thermal cracking" is generally applied to the pyrolysis of distillates heavier than gasoline. Thermal cracking facilities now principally process cycle oils from catalytic cracking. As in other pyrolytic processes the feedstock is only partially converted in a single pass. The furnace effluent is quenched and the products are separated into gases and various fractions of liquid products.
When reduced crudes are pyrolyzed without substantial coke production, the process is called visbreaking. Because residual stocks have greater coking tendencies than gas oils, lower temperatures are employed in visbreaking than in thermal cracking. Typical reaction conditions are about 900.degree. F. and 250 psi, and recycle is not usually employed.
Coking is similar in principle to visbreaking except that more severe conditions are used and the reduced crude is completely converted to lighter and heavier materials. Coking has the advantage over visbreaking in that it produces more distillates from a given charge. Coking temperatures may range from 930.degree. F. to 1050.degree. F.